The invention relates to well screens, whose principal purpose is to restrain sand, gravel or other particulate matter from entering water pumps and distribution systems. Most screens capable of restraining fine "flour" sands are made by the spiral wrap process, in which a continuous wire is helically wound about an internal cylindrical supporting structure. Each turn of the wire is separated from the preceding turn by a gap, or "slot" whose width determines the size of the particles restrained.
In spiral wrapped metal screens of the type disclosed in Johnson U.S. Pat. No. 2,046,058, the inner structure is a plurality of rods axially aligned and radially disposed and attached to the inner surfaces of the enwrapment. The enwrapping wire and the slot adjacent thereto are both continuous. The inner rods offer little or no obstruction to inward flow, thus maintaining high slot efficiency. The spiral wrap process has proven to be the most efficient system of fine slot generation.
In recent years the cost, scarcity and processing expense of metals for casing and screens has spurred the use of synthetic polymers as substitutes for metals. It is known in the arts that the modulus of elasticity of the commodity priced plastics range from 300,000 to 500,000 psi. Since this is roughly 1/60th of the stiffness of steel and its alloys, a facsimile reproduction of a steel product in plastics will result in a far weaker product, possibly unacceptable for equivalent usage.
The many plastic screens available divide in two general groups: 1. Modification of bare pipe by saw slots, grinding, drilling or milling to produce water passages. Usually this group of screens have no reverse draft to facilitate the passage of small particulate matter to reduce blockage. 2. Composite screens. In this group, a pipe slotted or drilled to provide passage is overwrapped with plastic wire to generate fine slots. Most screens of this type have very low effective slot area, one exception being the screen disclosed in U.S. Pat. No. 3,211,819 which comprises a wire wrapped around a perforated tubular core.